The invention relates to an intelligent spring. More particularly, the subject of the present invention is a spring equipped with a deformation sensor for measuring a load on a wheel of a vehicle. This type of spring has many applications in the field of vehicles. Knowing the load being applied to the wheels of a vehicle constitutes input data that can be used for controlling the suspension, for matching the computer-controlled electro-stabilizer, for optimizing the braking, or even for warning the user of a vehicle in the case of an overload.
In this field, means are already known that allow the load being applied to a wheel of a vehicle to be estimated. These means consist of devices of the deformation gauge type, also usually referred to as strain gauges.
The aim of this gauge is to translate the deformation of a component into a variation in electrical resistance: the more the strain gauge is stretched, the higher the resistance.
This gauge is bonded onto the spring. Thus, when strain forces are applied to the spring, a variation in the value of the resistance will be measured subsequently allowing the variation in length to be deduced from this value. Using the variation of the length, it is then possible to deduce the strain being exerted on the spring and thus to deduce from this the force being exerted on the spring. The load being exerted on the wheel of the vehicle is deduced from this.
The conditions of use of such gauges have several drawbacks.
Since it is bonded onto the support, this being the spring, the strain gauge is disadvantageously subjected to the environmental conditions of the vehicle.
More precisely, the test body defining the part that will undergo the deformations is composed of a material for which it is difficult to find a compromise. For example, certain steel alloys offer a good measurement precision and an excellent resistance to fatigue but are vulnerable to corrosion. In contrast, stainless steel exhibits a good resistance to corrosion but it is less homogeneous and is hence less precise.
Furthermore, the gauge is subjected to other undesirable effects such as thermoelectric effects linked to the difference in temperature between two connection points of the gauge.
Another undesirable effect that strain gauges exhibit is a hysteresis phenomenon, in other words the information that the gauge delivers is different depending on whether the measurements are under an increasing or decreasing load. At the present time, it is not possible to compensate for this with the measurement electronics. This phenomenon is notably dependent on the material composing the strain gauges.
Another drawback characterizing this type of measurement solution is that the information delivered at the output is not always proportional to the input value, which is detrimental to the quality of the measurement.